This disclosure relates to self-dispersing pigment dispersions and particularly to their use in ink jet inks.
Aqueous dispersions of pigments are known in the art and have been used in various applications, such as, inks for printing (particularly ink jet printing); waterborne paints and other coating formulations for vehicles, buildings, road markings and the like; cosmetics; pharmaceutical preparations; etc. Because pigments are typically not soluble in an aqueous vehicle, use of dispersing agents is often required, such as, polymeric dispersants or surfactants, to produce a stable dispersion of the pigment in the vehicle.
The use of dispersing agents, however, increases the viscosity of the dispersion over what it would be without the dispersing agents. The increase in viscosity, while not particularly a problem for some uses, presents a significant disadvantage in others. For example, viscosity limitations on the end use application of the dispersion (e.g., ink jet inks) may be exceeded if binders are added to improve properties of the final product. Thus, the viscosity of the dispersion itself will reduce the formulation latitude for final products.
Further because pigments are typically insoluble particulates, they have a propensity to settle if stored undisturbed for extended periods of time. Upon settling, these particles can cause clogging of nozzles and poor print quality. The recovery of these nozzles and print quality may be achieved by using printer-driven cleaning procedures. Alternately, they may require human intervention whereby the print cartridge is removed from the printer and vacuum applied to re-prime the print cartridge. Such servicing routines to recover print quality after storage are inconvenient and increase the cost of operation because of the loss of ink from the print cartridge. The severity of clogging and the subsequent ease of recovery of print quality are found to correlate to the ease with which pigments in the dispersion pack in proximity with each other. Easier packing of pigments leads to faster rate of increase of pigment density at the nozzle orifices during periods of no printing wherein the pens are stored with the nozzles facing downwards as is the normal configuration in a printer. Therefore, the faster the pigment densification rate, the more severe clogging of the nozzle orifices and the recovery of print quality is more difficult.
A need exists for ink jet inks to jet with ease and reliability after extended storage in the pen.